A human ear may be divided into three sections, an outer ear including auricle and external auditory meatus, a middle ear including tympanic membrane, auditory ossicies and Eustachian tube, and an inner ear including cochlea and vestibule. The inner ear encodes physical stimuli from outside into electrical signals. Within the inner ear, vestibule responsible for balance exists as a separate section from cochlea that houses end organ of hearing. Within the vestibule, semicircular canals for sensing angular acceleration and otolith organs for sensing linear acceleration exist. The otolith organs of each inner ear are comprised of sacculus for sensing vertical linear acceleration and utriculus for sensing horizontal linear acceleration.
Within a temporal bone, the otolith organs coexist with the semicircular canals. Taking utriculus as an example, it is explained below how otolith organ contributes to sensing of linear acceleration.
Utriculus is a sac filled with a fluid called endolymph. Within the thickened bottom of walls defining the sac, hair cells are embedded as being surrounded by supporting cells and associated with nerve endings to form a macula called an utricular macula (macula utricule). The gelatinous material that overlies the macula is called the otolith membrane. It contains tiny crystalline particles of calcium carbonate, called otoliths (otoconia). The specific weight of the otoliths is greater than that of the endolymph.
Linear acceleration movement of the human head causes the hair cells to move together with the head, causing the otolith membrane to slide, due to the inertia, over the hair cells, bending the hairs. When the hair cells are bent toward the kinocilium, the hair cells depolarize and impulses sent to the brain increase in frequency. When the hair cells are bent in the opposite direction, the receptors hyperpolarize and impulse generation declines. As a result, the utricular maculae respond to changes in linear acceleration or velocity of head movement within the horizontal plane. Since the macula of the sacculus (macula saccule) is disposed on the side of the sac, the saccular maculae respond to changes in linear acceleration or velocity of head movement within the vertical plane.
The structure of a macula has a surface so that the utricular maculae and saccular maculae contribute effectively to sensing of linear acceleration of head movement in the tangential direction to the surfaces (W. Precht: “The physiology of the vestibular nuclei. Part 1” In: Kormhuber HH, ed. Vestibular system. Berlin. Germany: Springer-Verlag. 1974: pp 353-416, Handbook of sensory physiology: vol 6). Accordingly, the total four maculae consisting of two on the left side of a human head and the other two on the right side contribute to sensing the tilted position of the head on the recognition of the gravity's direction (vertical linear acceleration).
A great number of researches have been reported on otolith stimulation for an improved functional test of otolith organs. Representative examples of various proposals derived from the researches are an otolith stimulation by linear acceleration, an otolith stimulation during parallel swing, an otolith simulation by tilting the support surface, an otolith stimulation during dynamic rotation to induce binocular counterrolling, an otolith stimulation during off-vertical axis rotation, and an otolith stimulation during eccentric rotation. These stimulation methods are briefly described below.
(1) Otolith Stimulation by Linear Acceleration:
It is reported that sinusoidal linear acceleration of the head movement to the left or right stimulated the utriculus maculae in left and right ears to induce horizontal compensatory eye movements (Akito FUJINO: “Complementary eye movements due to stimulation by sinusoidal linear acceleration—Eye reflex under otolith influence in normal humans vs., frequency characteristic and functional characteristic of otolith organs—”Ganjibi (Eye Ear Nose) 90: 335-347, 1987). As the left and right utriculus maculae are simultaneously stimulated by linear acceleration, this method is inappropriate for subjecting them to different stimuli.
(2) Otolith Stimulation During Parallel Swing:
The parallel swing can give sufficiently great movements of a subject within the horizontal plane with small movements within the vertical plane. A trial is reported to determine the function of otolith organs by stimulating humans and animals during the parallel swing in the direction of axon (head-to-tail) and/or from side to side (H. J. Scholtz: “Kompensatorische Augenbewegungen auf der parallelschwingenden Horizontalschaukel bei Gesunden und Vestibulariskranken” Z. Larying. Rhinol. 51: 46-57, 1972). As the utricular maculae and saccular maculae in the left and right ears are simultaneously subjected to liner acceleration, this method is inappropriate for subjecting them to different stimuli.
FUTAKI et. al. (Takashi FUTAKI & Isuzu KAWABATA: “Morphological study of otolitics of a guinea pig during parallel swing” Jibirinsho (Oto-rhino clinical practice) 75: Reprinting 5: 2468-2476, 1982) discloses measuring eye movements of a guinea pig with its head set in right position during swing over 45 degrees in head-to-foot direction. The utricular maculae of both ears are simultaneously stimulated during the parallel swing. Accordingly, it is not considered that this method is appropriate for subjecting the utricular maculae to different stimuli.
(3) Otolith Stimulation by Tilting the Support Surface:
For examination of the reflex of labyrinth (labyrinthine righting reflex) and the reflex of neck (righting neck reflex) to enable a human to maintain the equilibrium, this stimulation method is proposed. As the head is tilted to stimulate the otolith organs for investigation of labyrinthine righting reflex, this method is inappropriate for subjecting the left and right utricular maculae to different stimuli (Masaaki KITAHARA: “Acceleration registography—A new method of examination concerned with the labyrinthine righting reflex—”Ann Otol 74: 203-215, 1965).
(4) Otolith Stimulation During Dynamic Rotation to Induce Binocular Counterrolling:
This stimulation method, by which binocular counterrolling is measured during dynamic rotation of the subject with the head tilted, stimulates the saccular maculae or the urticular maculae simultaneously (Shireley G. DIAMOND et. al.: “Binocular counterroling in humans during dynamic rotation” Acta Otolaryngol 87: 490-498, 1979). As the utricular maculae are simultaneously stimulated, this method is inappropriate for subjecting the utricular maculae to different stimuli.
(5) Otolith Stimulation During Off-vertical Axis Rotation (Off-vertical Axis Rotation; OVAR):
Included in this category are two methods, namely, a “rotation then tilt method” that tilts an axis of a chair, with respect to the direction of the gravity, after a stable state of the rotation about the axis has been established, and a “tilt then rotation method” that tilts the axis of the chair before the chair is driven to rotate. In either stimulation method, as the direction of the gravitational force applied to the head of the subject varies with different angular positions of the chain, the otolith organs are stimulated to induce eye movements due to otolith-eye reflex. As will be noted, these methods simultaneously stimulate the utricular and succular maculae within the both ears in a similar manner to the various stimulation methods discussed above (C. DARLOT, P. DENISE, J. DROULEZ, B. COHEN, AND A. BERTHOZ: “Eye movements induced by off-vertical axis rotation (OVAR) at small angles of tilt” Exp Brain Res (1988) 73: pp 91-105).
(6) Otolith Stimulation During Eccentric Rotation:
To stimulate the vestibule, a subject is placed on the axis of rotation (earth vertical axis: EVA) of a chair with the horizontal semicircular canals held within the horizontal plane parallel to the surface of earth to achieve rotation for observation of the vestibular-ocular reflex (VOR) gain in the concentric position. In the concentric position, the eye movements induced by the semicircular canals-ocular reflex occur. When the subject is placed off from the axis of rotation to achieve eccentric rotation, the subject receives not only angular acceleration forces, but also linear acceleration forces (tangential and normal acceleration forces). Under this condition, the otolith organs and the semicircular canals are simultaneously stimulated to give the total eye movements resulting from adding the eye movements induced by the otolith-ocular reflex to the eye movements induced by the semicircular canals-ocular reflex. The otolith function can be recognized by comparing the eye movements observed in the concentric position to the eye movements observed in the eccentric position. According to this stimulation method, however, the semicircular canals and the otolith organs of both ears are simultaneously stimulated (Izumi KOIZUKA, Noriak-i TAKEDA, Shinji SATO, Takeshi KUBO & Toru MATSUNAGA: “Nystagmus Responses in Normal Subjects during Eccentric Sinusoidal Rotation”: Acta Octolaryngol (Stockh) 1993; Suppl. 501: pp 34-37).
As, with the previously listed systems, failure to achieve applying left and right utricular or saccular maculae with different stimuli makes individual functional tests impossible, the difficulty to accomplish the desired accuracy of the individual functional tests remains. If a system for applying the maculae with different stimuli were developed, the accuracy of the individual functional tests of the otolith organs would be enhanced, making a great advancement in this field of the medical treatment. Regrettably, there is no report on development of such system.
The inventor of the present invention has been involved in the functional test and study for many years. To the best knowledge of the inventor, there is no report on the significant progress in three-dimensional analysis of morphological aspects of the human otolith organs in the basic morphological study of the human otolith organs. Accordingly, as shown in FIG. 1(A), the recognition that the maculae of saccule and utricule are oriented vertically and horizontally, respectively, remains as the common knowledge (Kyoya. NOMURA, Fumihiko HIRAIDE & Takehiro HARADA: “New Otol Science Atlas-Morphology and Measure” pp 176: Sanshodo Printing Co., Ltd., Japan).
Among reports on the morphology of human otolith organs, Hiroshi SASAKI reported on the relationship, with respect to the a German horizontal plane (a flat plane containing the upper ends of external auditory meattus and the infraorbital region) and the sagittal plane, of three planes, namely, a major plane, a front plane and a rear plane, which the saccular macula was said to be composed of The utricular macula was said to be composed of three plane, namely, a front plane, a major plane and an inner plane. He reported that the front side of the major plane was lifted about 12° from the German plane and oriented inwardly about 90° from the sagittal plane, and the outer side of the major plane is lowered about 10° from the German plane (Hiroshi SASAKI: “Otolith organs basic science and clinical practice” Oto-rhino clinical practice 60, Suppl. 2: 1970 pp 73-123).
Over recent years, the inventor was devoting himself to the study of precise three-dimensional analysis of human otolith organs, more particularly, morphological aspects of succular macula, and reported the results in the year of 2001. In this report, a reference plane in the Reid stereotaxic coordinate system was determined by calculation. Using this calculated reference plane, each of multiple elements of the saccular macula (the multiple elements being several hundreds of microtriangular planes resulting from dividing the saccular macula) was evaluated in terms of its angular relationship with respect to the anterior-posterior, the left-right, and superior-inferior axes of the human skull. The results of the report demonstrated that the overall surface contour of the saccular macula was (not composed of flat planes) a curved surface forming a part of the surface of an ellipsoid (Hideaki NAGANUMA, Koji TOKUMASU, Makito OKAMOTO, Shinichiro HASHIMOTO, and Shohei YAMASHINA: “Three-dimensional analysis of morphological aspects of the human saccular macula”, Ann Otol Rhinol Laryngol 110-2001, pp 1017-1024).
Using the same three-dimensional analysis as that used for the saccular macula, the inventor was devoting himself to the study of the human utircular macula over long years. As a result, it was made clear that the overall surface contour of the human utricular macula was not composed of flat planes, but a curved surface forming a part of the surface of an ellipsoid. It was also made clear that the exterior side of the utricular macula is tilted down about 10° from the horizontal plane (a flat plane containing three points including the upper ends of external auditory meattus and the infraorbital region) of the skull in the Reid stereotaxic coordinate system [see FIG. 1(B)]. This result confirmed the report by SASAKI (Hiroshi SASAKI: “Otolith organs basic science and clinical practice” Oto-rhino clinical practice 60, Suppl. 2: 1970 pp 73-123).
From the preceding three-dimensional analysis, it is now apparent that the exterior side of the utricular macula is tilted down about 10° from the horizontal plane (a flat plane containing three points including the upper ends of external auditory meattus and the infraorbital region) of the skull in the Reid stereotaxic coordinate system [R. H. I. BLANKS, I. S. CURTHOYS and H. MARKHAM, “Planar relationships of the semicircular canals in man”, Acta Otolaryngol 80: pp 185-196 m 1975].